The present invention relates to optical device technique including an optical waveguide and a microoptical circuit.
The coupling efficiency of two optical waveguide modes is in proportion to the superposition of the distribution of an electric field (hereinafter called a mode field) in a plane perpendicular to a traveling direction in each optical waveguide mode. Therefore, in case the size of a mode field of the two optical waveguide modes or each distribution pattern is different, the coupling efficiency is small. Coupling efficiency is increased by converting the size and the pattern of either mode field so that it is equal to those of another optical waveguide mode.
Some well-known examples using the method exist. (1) There is an external device coupled waveguide of a low-loss Si thin line optical waveguide acquired by varying the size of the sections of a core in a shape reverse to a taper as described in a document of the 48th Applied Physics Related Association Lecture Meeting 30a-YK-11. In the case of the reversely tapered core, the size of a mode field is made larger by reducing the cross section of the core of the optical waveguide. Or the size of the mode field is reduced by increasing the cross section of the core of the optical waveguide. (2) There is an optical waveguide composed of a tapered core and a clad layer formed by metal or photonic crystal material as described in Japanese Patent Application Laid-Open No. 2001-4887. In the case of the tapered core, the size of a mode field is reduced by reducing the cross section of the core of the optical waveguide. Or the size of the mode field is made larger by increasing the cross section of the core of the optical waveguide. (3) For a device using an optical fiber described on the 52nd page, vol. 13 of IEEE Photonic Technology Letters published in 2001, there is a mode field converting device in which a clad layer made of an airy silica fiber and a core layer are both tapered. (4) The 358th page of proceedings C-358 of the Institute of Electronics, Information and Communication Engineers General Meeting (1995) discloses optical waveguide structure in which a mode size converter of a semiconductor laser provided with the mode size converter is provided with both a tapered core and a reversely tapered core. (5) In Japanese Patent Application Laid-Open No. 2001-4869, an optical coupling device that varies spot size in a different class utilizing chromatic dispersion proper to a photonic crystal is proposed.
On the 235th page, vol. 136 of IEEE proceedings written by J. D. Love and published in 1989, a conditional expression which is the criterion of judgment not to change field distribution when the size of a mode field is varied is described. The conversion of only the size of a mode field without changing field distribution is called adiabatic mode conversion.
On the 1080th page, vol. 9 of Microwave and Optical Technology Letters (1994), a method of spatially controlling the thickness in a direction of crystal growth of a semiconductor using selective crystal growth is described.
Next, in case light is incident on the interface of two media different in a refractive index, reflection is caused. To reduce the reflection, an antireflection film is provided. It is known that to eliminate reflection caused in case light is incident on a medium 2 having a refractive index n2 from a medium 0 having a refractive index n0, a medium 1 having a refractive index n1 and thickness L which meets the following conditions has only to be inserted between the media 1 and 2 (refer to the 294th page in sixth and eighth chapters of the second edition of “Field and Wave in Communication Electronics” written by Simon Ramo et al and published by John Wily and Sun Press).n12=n0·n2  (1)k1·L=π/2  (2)In the case, k1 denotes the wave number of light in the medium 1.
A microoptical circuit using a photonic crystal optical waveguide and a microguide optical waveguide attracts attention.
A photonic crystal means periodic structure having a period equivalent to the wavelength of light and formed by two types of media having much difference in a refractive index. A photonic crystal waveguide in which the intense confinement of light is enabled can be produced by injecting a defect into periodicity. For the photonic crystal optical waveguide, there are a line-defect waveguide in which defects are physically connected and a coupled-defect waveguide in which detects are optically connected though detects are not physically connected. In such an optical waveguide, even if the waveguide is greatly curved, an optical propagational loss (a flexural loss) by flexure is very small. As a result, it is expected that if the optical waveguide is used, a very small optical circuit is enabled.
The microguide optical waveguide means an optical waveguide having much difference in a refractive index between a core and a clad of the optical waveguide. Normally, as the difference in a refractive index between the core and the clad is 1 or more and the confinement of light is intense, the flexural loss is small as in the photonic crystal optical waveguide and the microguide optical waveguide is expected as a microoptical circuit.
When the optical waveguide in which the confinement of light is intense is supposed to be a single mode optical waveguide in which only a fundamental guided mode exists, its mode field diameter is 1 μm or less. In this case, the fundamental mode means an optical waveguide mode in which a pattern of electric field strength has s single peak. The mode field diameter is defined as a diameter of an electric field region including 1/e2 which is the maximum value of the electric field strength distribution of a mode or more.
A single mode optical fiber is used for the input/output of light to/from an optical circuit. An optical fiber means an optical waveguide having concentric structure provided with a core in the center, a clad around the core and further, a protective layer around the clad. The single mode optical fiber means an optical fiber in which only one optical waveguide mode of a fundamental mode exists. Currently, for a medium for transmitting an optical signal of an optical communication system, the single mode fiber is used. In the single mode fiber, as difference in a refractive index between a core and clad is small, the confinement of light is weak and as a result, the existing mode field diameter of the fundamental mode is approximately 10 μm.
Therefore, in case light is directly input to the microoptical circuit provided with the photonic crystal optical waveguide or the microguide optical waveguide from the single mode fiber or in case light is directly output to the single mode fiber from the microoptical circuit, a large coupling loss is caused because there is much difference between the mode field diameter of the single mode fiber and the mode field diameter of the microoptical circuit. In addition to the coupling loss caused by the difference in the size of the mode field diameter, a coupling loss by reflection on a boundary caused by difference in a medium between the optical fiber and the microoptical circuit also exists.
For a method of reducing the coupling loss caused by the difference in the mode field diameter, there are the methods described in (1) to (5) of the prior art. The methods (1) to (4) include plural complex working processes for forming the tapered core, forming the buried waveguide and forming the film. In the methods (2) and (3), as the tapered core is provided, plural waveguide modes exist as the diameter of the core is increased. Therefore, in case light is input to the core the diameter of which is large from the single mode fiber, a higher order optical waveguide mode is excited in addition to the fundamental guided mode and a new coupling loss is caused. In (5), in addition to the complexity of working, the difficulty of optical alignment is also caused.